An organic electroluminescence display (hereinafter referred to as “organic EL display”) is an application of the phenomenon that an organic thin film having a thickness of about 1 μm emits light when a current is injected into the organic thin film, and it has been actively researched and developed in recent years. A typical structure of the organic EL display is a layered structure including an output-side substrate 10′, a reflection-side substrate 20′ and an organic EL light-emitting layer 30′ interposed between the substrates 10′ and 20′, as illustrated in FIG. 7. The output-side substrate 10′ includes an output-side substrate body 11′, a transparent electrode 12′ made of ITO (Indium Tin Oxiside) provided on the inner side of the output-side substrate body 11′, and a hole injection/transfer layer 13′ provided on the inner side of the transparent electrode 12′. The reflection-side substrate 20′ includes a reflection-side substrate body 21′ and a metal electrode 22′ made of aluminum. In an organic EL display having such a structure, a portion of light from the organic EL light-emitting layer 30′, which emits light omnidirectionally, that travels toward the output-side substrate 10′ is output directly from the output-side substrate 10′, while another portion of the light that travels toward the reflection-side substrate 20′ is output indirectly from the output-side substrate 10′ after being reflected by the metal electrode 22′ having a mirror surface, thus efficiently taking out the light emitted from the organic EL light-emitting layer 30′.
An organic EL display has a problem as follows when it is used under the sunlight or in the presence of room light, as is a portable telephone, or the like. That is, when ambient light such as the sunlight or room light enters the organic EL display through the output-side substrate, the ambient light is reflected by the metal electrode and is output through the output-side substrate, and the contrast of the organic EL display is lowered significantly by the ambient light reflection.
To address this problem, Japanese Laid-Open Patent Publication No. 8-321381 discloses the provision of a circular polarization plate including a linear polarization plate and a retardation plate on the transparent electrode side of an organic EL display, and Japanese Laid-Open Patent Publication No. 9-127885 discloses a light emitting device in which circular polarization means including a linear polarization plate and a ¼ wave plate is provided on the light-outputting surface side. According to the disclosure of these publications, half of the ambient light is blocked by a linear polarization plate. The linearly-polarized light of the remaining half of the ambient light transmitted through the linear polarization plate is turned into circularly-polarized light (e.g., right-handed circularly-polarized light) by a retardation plate or a ¼ wave plate, and then passes through a transparent electrode, after which it is turned into circularly-polarized light of the reverse direction (right-handed circularly-polarized light being turned into left-handed circularly-polarized light) as it is reflected by an aluminum electrode. Then, the circularly-polarized light of the reverse direction is turned into linearly-polarized light by the retardation plate or the ¼ wave plate. However, the polarization axis of the linearly-polarized light has been rotated by 90° with respect to that of the original linearly-polarized light, whereby the linearly-polarized light is blocked by the linear polarization plate. Therefore, all of the ambient light incident on the organic EL display is blocked by the linear polarization plate, thus preventing the reflection of the ambient light from entering the viewer's eye, thereby preventing the contrast from being lowered by the ambient light reflection.
On the other hand, in an organic EL display provided with a linear polarization plate and a retardation plate or a ¼ wave plate, half of the light from the EL light-emitting layer is also blocked and lost by the linear polarization plate, thereby reducing the luminance by half, as compared to a case where these members are not provided, and such an organic EL display will require twice as much power as an organic EL display where these members are not provided in order to obtain an equal level of luminance.
To address this problem, Japanese Laid-Open Patent Publication No. 2001-35653 discloses a filter for an organic EL panel including a ¼ wave plate, a polarization plate and an antiglare layer, wherein the degree of polarization of the polarization plate is 50 to 70%, and states that it is possible with such an arrangement to obtain an organic EL panel with a high visibility that is glare-free and capable of displaying sharp images.
Moreover, Japanese Laid-Open Patent Publication No. 4-218025 discloses a reflection type liquid crystal display device including a liquid crystal cell, including a pair of substrates opposing each other and a liquid crystal material interposed therebetween, a polarization plate provided on each side of the liquid crystal cell, and a reflection plate provided on one side of the liquid crystal cell, wherein a polarization plate having a low degree of polarization is used as the polarization plate on the reflection plate side, thus improving the overall brightness.